This invention was made with Government support under Contract DE-AC0676RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
The present invention relates generally to method for manufacturing lanthanum oxide containing solids in an aqueous environment. More specifically, the present invention relates to a method of forming a salt layer surrounding lanthanum containing solids in an aqueous environment, thereby allowing further processing of the materials without dissolution.
Lanthanum oxide containing solids, including but not limited to lanthanum gallates, lanthanum cobaltites, lanthanum manganites, lanthanum chromites, lanthanum ferrites, and combinations thereof, have been shown to be useful in a variety of industrial processes. For example, Lanthanum gallate has proven to be useful as a solid electrolyte material in the production of solid oxide fuel cells (SOFC). Unfortunately, these materials have poor stability in an aqueous environment. For example, in water, lanthanum gallate incongruently dissolves to La3+ and Ga2O3 by the following reaction:
2LaGaO3(s)+3H2O2La3+(a)+6OHxe2x88x92(a)+Ga2O3(s).xe2x80x83xe2x80x83[1]
This reaction in turn produces a large amount of hydroxyl ions which drives the pH of the solution up into a region where Ga2O3 becomes unstable ( greater than pH 12). The increased number of hydroxyl ions also react with La3+ to form La(OH)3 beginning at pH 8. This combination of events renders the materials useless for further processing.
The solubility of these materials in water has led those skilled in the art to process the materials in non-aqueous environments. Unfortunately, non-aqueous processing typically requires the use of solvents that are biologically harmful and generally not environmentally friendly. However, in industrial and large scale manufacturing of these materials, the costs associated with environmentally safe disposal of these solvents, and the costs associated with safety procedures required for handling these solvents under current regulatory schemes, can often prove to be prohibitively expensive.
Thus, there exists a need for a method and techniques for processing lanthanum oxide containing solids in an aqueous environment.
The present invention allows lanthanum oxide containing solids to be processed in an aqueous environment by forming a partially soluble or insoluble salt coating around the lanthanum oxide containing solid in an aqueous environment. The present invention is a method whereby an aqueous solution containing a partially soluble or insoluble salt forming anion is provided. A soluble lanthanum oxide solid is then added to the solution, and the anion is then reacted with at least a portion of the lanthanum in the lanthanum oxide solid, thereby forming a partially soluble or insoluble salt coating around the lanthanum oxide solid.
Preferably, and not meant to be limiting, between ten and one hundred times more anions than lanthanum cations are provided to ensure sufficient anion presence for salt formation. In theory, the concentrations of anions required depends on the lanthanum oxide solubility. Since the solubility product of each variation of lanthanum oxide will be different, the specific amount of anion necessary to react with the free lanthanum will vary. To calculate the minimum amount of anion needed to react with a given amount of lanthanum in solution, the solubility product (Ksp) of the soluble lanthanum complex oxide is first determined. The theoretical minimum amount of anion then needed may be calculated by taking the square root of the Ksp, multiplying by the absolute value of the valence of lanthanum (3) and dividing by the absolute value of the valence of the reacting anionic species according to the following equation:
Minimum Anion Concentration to Passivate=(Ksp){circumflex over ( )}(xc2xd)xc3x973/|anion valence.
While a great many anions are useful in the practice of the present invention, including but not limited to, Clxe2x88x92, ClO3xe2x88x92, Brxe2x88x92, NO3xe2x88x92, SO42xe2x88x92, PO43xe2x88x92, CO32xe2x88x92, C2O42xe2x88x92, C2H3O2xe2x88x92, and C6H5O73xe2x88x92, it is preferred that the anion be selected from the group consisting of phosphate, carbonate, oxalate and combinations thereof.
The present invention is useful in the processing of any lanthanum oxide solids, including, but not limited to, lanthanum gallates, lanthanum cobaltites, lanthanum manganites, lanthanum chromites, lanthanum ferrites, and combinations thereof.
The present invention is particularly useful for ceramic powder processing, which is a very important aspect of solid oxide fuel cell development, and more particularly for processing lanthanum gallate in the manufacture of solid oxide fuel cell electrolyte material.
Accordingly, it is an object of the present invention to provide a method processing lanthanum oxide containing solids in an aqueous environment.
It is a further object of the present invention to provide an aqueous solution containing a partially soluble or insoluble salt forming anion, adding a soluble lanthanum oxide solid to the solution, reacting the anion with at least a portion of the lanthanum in the lanthanum oxide solid, thereby forming a partially soluble or insoluble salt coating around the lanthanum oxide solid.